Important climate-sensitive parameters such as water
exchange, freshwater discharge and sedimentary patterns can be quantified at
very high resolution in fjord environments as sedimentation rates exceed
several mm/yr. The Trondheimsfjorden is located at the west coast of Mid-Norway
and is characterized by local environmental and hydrological changes that are
tightly reflecting regional oceanographic and atmospheric processes in the Norwegian Sea. The North Atlantic Current (NAC) and the
Norwegian Coastal Current (NCC), two major northward flowing sea surface/intermediate
currents, strongly contribute to the oceanography of the Norwegian
Sea and thus to the hydrological settings of the
Trondheimsfjorden. Previous
investigation (Rühlemann et al., 2008; EGU Fall Meeting) suggested that
oxygen isotope ratios in shells of the benthic foraminifera Melonis Barleanum of the
Trondheimsfjorden record past changes in bottom water density, directly linked
to the intensity of the NAC and NCC during the late Holocene. In order to
verify this assumption, 60 surface sediment samples, evenly distributed across
the Trondheimfjorden, have been obtained and analysed for oxygen isotope ratios
of Melonis Barleanum. We use this “modern” database as well as a
composite (multi-core and piston core) sedimentary archive recovered from the
center of the fjord in order (1) to test the validity of the oxygen isotope
ratio as a proxy of bottom water density, and (2) to provide detailed
information on hydrographic changes in the research area throughout the past
3,175 years BP. The stratigraphy of the composite core is based on 10 14C dates
as well as on 210Pb and 137Cs measurements, leading to sedimentation rates
of 2.5 - 4 mm/yr. The oxygen isotope ratios derived from Melonis Barleanum in the surface sediment samples clearly indicate
shifts according to the sample locations in the fjord and can be mainly related
to local factors such as distance to the fjord entrance, current patterns as
well as river inlets. The late Holocene downcore record shows a trend to
heavier d18O values from ca. 2,000 years BP, with high amplitudes changes,
followed by a decrease in the stable isotope signal within the last 300 years.
We interpret these drastic changes in bottom water density as a reflection of
variable dynamics of Atlantic-derived water in the Norwegian
Sea, which directly affect the ventilation of the
Trondheimsfjorden.